Electrical machines include, but are not limited to, rotary generators and motors, and linear generators and motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor and stator may include a set of coils and an iron core to carry magnetic flux. Conventional copper windings are commonly used in the rotors and stators. However, the electrical resistance of copper windings, though low by conventional measures, is sufficient to contribute to substantial heating. This can lead to diminished efficiency, and specific power and power density (because of thermal constraints).
Recently, superconducting (SC) coil windings have been developed. SC windings have effectively no resistance and are highly advantageous, especially for the substantially “DC” field windings. The primary reason for this is the difficulty in managing AC losses in SC coils. Furthermore, even “merely” replacing the DC field coils can already lead to a significant jump in performance by relieving many traditional tradeoffs. Several types of electrical machines employing superconducting coils have been developed.
SC coils are capable of much higher current density than conventional coils, enabling machine designs with much higher magnetic reluctance and higher magnetic fields than traditional machines. Superconducting machines may employ “air-core” designs, with no iron in the rotor, or use a highly saturated core, to achieve air-gap magnetic fields higher than 2 Tesla. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in size and weight of the machine.
With the iron and its saturation limit removed, designs with much higher air-gap flux densities can now be considered. Fields that are 5-10× the typical values in electrical machines are routinely used in other superconducting applications like NMR and MM magnets, and even higher fields are theoretically possible with superconductors However, the practical challenge of containing the magnetic field emanating from the machine needs to be overcome. This can be a problem when such superconducting machines are integrated within systems that are sensitive to magnetic fields, for example in airplanes that have sensitive electronics. Personal safety is also a consideration. For example, medical implants like pace-makers are designed to operate below a magnetic field of 0.0005 Tesla.
Three methods have been used in the past to minimize the field emanating from the machine:
(a) Space out the field coils very far from other equipment. This may not be very practical because a large distance is needed to let the field decay to a low enough value (e.g. the 5 Gauss or 0.0005 T limit for medical implants). Increasing pole count (reduced pole-pitch) will help reduce outer field, but is also detrimental to the flux coupling within the machine. Studies show that pole counts of higher than 6-8 are not practical in air-core designs.
(b) Use an eddy current shield in an outer armature design. GE utilized such an approach in the 1980's to minimize weight. This however is also not very practical, because of the very large eddy current losses in the shield.
(c) Use a passive magnetic shield. This approach has been the most popular among SC machine designers. Though this option is more practical than the above two, it leads to relatively low power density, not just because of the weight of the iron, but because design optimization would lead to a relatively low operating field to minimize amount of iron required to shield the field. In fact, most practical designs have modest operating magnetic field levels (˜2T).